Your question is way, way, way, too broad. First of all, you don't need APIs to import data, you can simply use Excel. And Accpac 5.2 is totally out of date.

Data dictionary is here: partners.sagenorthamerica.com/.../210-30254.html

Thanks for reply Jay, But I just need to do some automation process. Does it possible with the inbuilt any sage tool?

You can record a macro and take it from there. How much automation do you need?

Start by recording a Sage VBA macro that adds a customer record , then wrap that code with your own VB.Net code. Your code will have to login with something like this:

Session = New AccpacCOMAPI.AccpacSession
Session.Init("", "XY", "XY0001", "53A")
Session.Open(My.Settings.AccpacUser, My.Settings.AccpacPW, My.Settings.Company, Date.Today, 0, 0)
a4wLink = Session.OpenDBLink(tagDBLinkTypeEnum.DBLINK_COMPANY, tagDBLinkFlagsEnum.DBLINK_FLG_READWRITE)
a4wLinkRead = Session.OpenDBLink(tagDBLinkTypeEnum.DBLINK_COMPANY, tagDBLinkFlagsEnum.DBLINK_FLG_READONLY)

It's on going process, it is not one time process.

Try being more cryptic, it helps us guess what you are attempting to do.